Transmission line windows



June 12, 1962 A. GRECO 3,039,068

I TRANSMISSION LINE WINDOWS Filed Aug. 5, 1960 2 Sheets-Sheet 1 .INVENTORI NORMAN A. GRECO,

HIS ATTORNEY.

United States Patent 3,039,068 TRANSMISSIGN LINE WINDiOWS Norman A. Green, Menlo Park, Calif, assignor to General Electric Company, a corporation of New York Filed Aug. 5, 1960, Ser. No. 47,757 7 Claims. (Cl. 333-98) This invention relates to barriers for pressurized or evacuated waveguides and the supporting structure for such barriers. That is, the invention relates to barrier sections which separate two regions in a waveguide or waveguide system and provide for the transmission of electromagnetic waves between the two regions. The barriers or seals of the type under consideration are called microwave windows.

The use of microwave windows introduces a number of electrical and mechanical problems. Electrical problems include the introduction of reflections at the dielectric barrier due to the discontinuity in the medium in which the electromagnetic Waves must travel, voltage breakdown in the presence of high electric and magnetic fields in the area of the dielectric barrier and the dissipation of microwave power within the dielectric material. The mechanical problems encountered include difiiculty in providing a leak-proof mechanical design which can withstand the elevated temperatures required for processing and operation (may be as high as 600 C.), providing the mechanical strength to maintain transmission line dimensions at correct values and providing for the reproduction in quantity with uniformity in dimensions and properties.

The electrical problems involved may best be understood by considering an application for which microwave windows embodying the present invention are particularly suited. As an example, consider a window for the transmission of microwave energy from a high powered source such as a klystron to an external waveguide system. An output window for such generators should be capable of transmitting the generated power to the waveguide system with a minimum of reflection and absorption of energy. Power absorbed at the window or reflected by the window is lost and that energy which is reflected may severely damage the power source. That energy which is absorbed in the window results in heat and thus a rise of temperature which in high power klystrons may reach a point where the window vacuum seal fails either through lowering of the breakdown voltage or by mechanical failure stemming from stresses created by differences in thermal expansion between the window dielectric material and the metal seal. In the window under consideration the problem of matching the impedance at the window or barrier so that the barrier itself does not represent an abrupt reflective discontinuity in the frequency range of interest is considered and the problem of providing a window design suitable for microwave frequencies which represents an improvement in the power handling capability devices of the present art is also considered.

The mechanical problems are easier to understand than the electrical problems. For example, it is easy to see that if the design of the window is not such as to permit a vacuum tight joint the barrier is ineffective. Also if the mechanical strength is not sufficient to maintain the dimensions both under normal operating conditions and at the elevated temperatures required for processing the advantages of a careful design are lost. Further, unless the design is capable of production and reproduction in quantity with uniformity in dimensions and properties the usefulness is reduced considerably.

A number of the schemes proposed for solving the problems outlined above involve dividing the microwave power into two or more waveguides prior to passing the electromagnetic energy through a window in order to "ice 2 reduce the amount of power incident on any one window. A separate window is provided in each waveguide. After passing through the microwave windows the microwave energy is recombined in a single waveguide. These units are extremely bulky and can easily exceed the tube in size. The mechanical design is intrinsically complicated thereby leading to prohibitive costs and unreliability. In addition, there is the electrical problem of re-combining the radio frequency energy from two or more waveguides having lengths which are not negligible in terms of electrical wavelengths in the microwave frequency range. This greatly complicates the radio frequency matching problem over the useful bandwidth.

It is an object of the present invention to alleviate these electrical and mechanical problems by providing a microwave window structure which divides the microwave power transmitted between a number of window elements without separating the power into a corresponding number of different waveguides.

Briefly stated in accordance with the present invention the microwave window is formed between two waveguide sections which provides for division of the electromagnetic power in such a manner that parts of the electromagnetic energy pass through different windows in one waveguide section and into a second single waveguide section.

The novel features which are believed to be characteristic of the invention are set forth in the appended claims. The invention itself, however, both as to its organization and method of operation together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawings in which:

FIGURE 1 is a broken away perspective view illustrating a window section constructed in accordance with the present invention;

FIGURE 2 is an end view of the window section of FIGURE 1 taken along the section lines 22;

FIGURE 3 is a vertical longitudinal section taken through the window section of FIGURES l and 2;

FIGURE 4 is an end view similar to that of FIGURE 2. of a window section having a slightly different construction;

FIGURE 5 is a graph which illustrates reflections at the windows of FIGURES 1, 2, and 3 and FIGURE 4 wherein frequency in kilomegacycles is plotted along the axis of the abscissas and the voltage standing wave ratio (VSWR) is plotted along the axis of the ordinates;

FIGURE 6 is a vertical longitudinal section taken through a different window section constructed in accordance with the present invention; and

FIGURES 7, 8 and 9' are partially broken away perspective views illustrating other embodiments of the invention.

Part of a waveguide system generally designated as a window section 10 is illustrated in FIGURES 1, 2, and 3. Since the waveguide systems joined by the window section 10 do not constitute part of the present invention, they are not illustrated. The window section 10 forms a waveguide section of rectangular cross-section composed of two ordinary waveguide sections 11 and 12. In order to provide a means for fastening the window section 10 to a waveguide system, a metal flange 13 is fixed on the free end of waveguide segment 11. The flange 13 is a substantially rectangular conductive piece with a centrally located aperture that fits snugly around the free end of waveguide segment Ill. The flange is fastened to the waveguide segment by some means such as brazing and is provided with a series of apertures 14 around its outer periphery so that the flange may be fastened to a similar flange on the waveguide system to which the window section 10 is to be connected. The waveguide segment 12 at the opposite end of the window section 10 is shown broken away but may be fastened directly to its associated waveguide system by any conventional means.

The barrier or seal is formed in the waveguide window section by the window supporting structure which closely resembles a gable roof. That is, the structure is formed by two essentially planar conductive window supporting plates 16 and 17 that slope inwardly from the broad walls of the Waveguide section 11 toward the center of the waveguide so that they meet at the center of the waveguide and two planar triangular conductive side plates 18 which are positioned between the window supporting plates 16 and 17 and close the internal angle formed between them. The triangular side plates 18 extend substantially parallel to the narrow walls of the waveguide 11. The window supporting plates 16 and 17 and the side plates 18 are made into a unitary structure which is vacuum tight by some means such as by forming brazes at all joints.

In order to provide for transmission of electromagnetic waves through the structure 15 the sloping window supporting plates 16 and 17 are provided with centrally located circular apertures 20 and 21 respectively which are filled with a dielectric material that is substantially transparent to electromagnetic waves. As illustrated, the dielectric material comprises two dielectric discs 22 and 23 inserted in the apertures 20 and 21 of each of the sloping window supporting plates 16 and 17. The discs 22 and 23 may be of any conventional dielectric window material such as high alumina ceramic or quartz. The discs 22 and 23 are sealed in vacuum tight relation in the apertures 20 and 21 by well known sealing techniques and the entire window supporting structure 15 is positioned and fixed inside the waveguide segment 11 in vacuum tight relation. As illustrated, the waveguide segment 12 is positioned over the window supporting structure 15 with its end flush against the end of the waveguide segment 11 and is fixed thereto by some method such as by brazing to complete the window section 10.

Since the window support structure 15 is vacuum tight and since it is sealed inside the waveguide segment in vacuum tight relation and is provided with two dielectric windows 22 and 23, the window section 10 is provided with a vacuum tight barrier which passes incident electromagnetic waves. Power incident on the window sup porting structure 15 is equally divided between the two windows 22 and 23. The method of construction described above is mostly a matter of convenience. The essential feature of the window is that incident power is divided between two windows instead of being passed through one window.

The embodiment of the invention illustrated in FIG- URE 4 is essentially the same as the embodiment illustrated in FIGURES 1, 2, and 3 with the exception that a dividing wall as septum 24 is provided between the two window discs 22 and 23, thus bi-secting the angle formed at the intersection of the two window supporting plates 16 and 17. Since the embodiment of FIGURE 4 differs from the previously described embodiment only in this one addition, the corresponding parts of the window sections illustrated in the two sets of figures are given corresponding reference numerals to simplify the description and drawings. The septum 24 constitutes a flat conductive plate which, as illustrated, extends the full length of the window supporting structure 15.

The characteristics of the two embodiments of the in vention just described are illustrated in the graph of FIGURE 5. The reflected power in terms of the voltage standing wave ratio (VSWR) is plotted along the axis of the ordinates. The voltage standing wave ratio represents the ratio of the maximum voltage (V measured along the waveguide, to the minimum voltage measured therealong, i.e.,

max. mln.

The VSWR is unity when there are no reflections in the guide, i.e., there is no standing wave in the waveguide and consequently the voltage is constant over the length of the guide. This represents the ideal condition. As is seen from the graph there are two curves plotted. One is plotted with a solid line and the other is plotted with a dash or broken line. The solid line curve represents the standing wave ratio for the window section without a septum, i.e., the window section of FIGURES 1, 2, and 3 and the broken line curve represents the VSWR of the window section of FIGURE 4 (the window section with a septum included). An inspection of the figure shows that a VSWR of 1.5 or less was obtained over a 15 bandwidth without the use of matching or tuning elements in the waveguides. Much larger bandwidths are obtainable using conventional matching elements or irises in the window section 10. The insertion loss figures indicate that the window is very good over the 15% passboard. A VSWR of 1.2 is considered by systems people to be excellent whereas tube people strike to obtain a VSWR on the order of 2.5 db. Generally, an insertion loss in the (passband which is less than 1.5 db is considered very goo Another embodiment of the present invention which is very similar to the embodiment illustrated in FIGURE 4 may be seen in FIGURE 6. This embodiment ditIers from the one illustrated in FIGURE 4 in only one respect. The diiference is that the septum 25 in the embodiment of FIGURE 6 extends through the hip of the window supporting structure 15 rather than being wholly contained between the two sloping window support plates 16 and 17. The septum 25 still extends entirely across the structure but its length is adjusted to obtain the best impedance match and to minimize ghost modes which may result from the use of any microwave window. In other words, the distance that septum 25 extends in between the two windows 22 and 23 and the distance that it extends on beyond the nose or apex of the gable roof-like window supporting structure 15 is adjusted for best impedance match and to minimize ghost modes at the barrier. Since the structure is so similar to the previously described structures all corresponding parts are given like reference numerals (except for septum 25) to simplify the description. Another embodiment of the invention which is almost ldentical to those previously described may be seen in FIGURE 7. Once again the parts of this window section 10 which correspond to parts previously described with respect to the other embodiments are given like reference numerals. In this embodiment, four disc shaped microwave windows 26 are utilized in each of the window supporting plates 16 and 17 instead of the single microwave windows 22 and 23 utilized in each of the window support plates 16 and 17 in the embodiment of FIGURES l, 2, and 3. The use of the additional windows further reduces the amount of power which must be handled by each individual window and provides additional opportunity for impedance matching. The microwave windows 26 again may be of any conventional material such as quartz, mica, or high alumina ceramic.

A somewhat diiferent variation of the same basic principle is utilized in the microwave window section illustrated in FIGURE 8. In this arrangement the microwave power is divided in rectangular waveguides 30 and 31 without the use of the sloping microwave window support plates 16 and 17 as utilized in all of the previous embodiments. In this arrangement a conventional rectangular waveguide 30 forms the support for a pair of dielectric microwave windows 32 and 33. The windows 32 and 33 are inserted and sealed in circular apertures provided on opposite broadwalls of the waveguide. The waveguide is made vacuum tight by utilizing a rectangular end plate or closure 34 over the open end. The second rectangular waveguide 31 is larger than the window supporting waveguide 30 and is positioned so that its longitudinal axis is coincident with that of the smaller guide 39 and one end surrounds the closed end 34 and windows 32 and 33 of the smaller waveguide 30. A rectangular end plate 35 with a rectangular aperture 36 therein acts as a seal between the two waveguides. The outer periphery of this rectangular end plate 35 fits the outer periphery of the larger waveguide 31 and the rectangular aperture 36 in the end plate 35 fits snugly around the outside of the smaller waveguide 30. T he end plate is sealed in vacuum tight relation to both waveguides 30 and 31. Thus, a vacuum tight barrier is provided between the two waveguides 3t and 31 and electromagnetic energy may be transmitted therebetween. Electromagnetic energy incident on the barrier is divided between and passes through the two microwave windows 32 and 33. The energy division is analogous to the energy division described in connection with the previous embodiments.

The embodiment illustrated in FIGURE 9 is very similar to that of FIGURE 8 and represents an extension of that embodiment. In FIGURE 9 the small rectangular waveguide 30 extends into the larger rectangular waveguide 31 just as in the embodiment of FIGURE 8. However, instead of utilizing one disc shaped dielectric window in each broad wall of the smaller waveguide 3t) there are four disc shaped dielectric windows 37. This arrangement has the advantage of reducing the power which must be handled by each individual window and provides an additional means of impedance matching.

It will be appreciated that all of the embodiments illustrated and described may utilize additional impedance matching structures such as irises and inductive posts to provide optimum impedance match at the barriers.

Objects of the invention are accomplished by providing a window and barrier section wherein the incident electro magnetic energy divides on one side of the barrier and passes in equal amounts through opposite sets of windows and after passing through the windows is combined because the electromagnetic waves are again in the same waveguide. Division of incident power between windows improved the power handling capability and the power recombination problem has been simplified because of the elimination of long line sections between the dividing and recombining of electromagnetic waves incident on the window section. Actually the length of the line between dividing and recombining of the incident electromagnetic waves is almost nil. Consequently, phase relations between wave-s emerging through the windows are proper for constructive interference in the outlet guide and avoidance of re-circulating currents. Energy may fiow in either direction without change in mode of operation. Further, the electrical impedance match over any design bandwidth is relatively straightforward because of the smooth natural transition from the evacuated to the pressurized side of the waveguide. In addition mechanical problems of complexity, bulk and weight have been substantially eliminated by the use of a single input and output waveguide which are almost identical to the coupling between two normal waveguide sections with standard flanges.

While particular embodiments of the invention have been shown and illustrated it will, of course, be understood the invention is not limited thereto since many modifications both in the circuit arrangement and in the instrumentalities employed may be made. It is contemplated that the appended claims will cover any such modification as fall within the true spirit and scope of the invention.

What I claim is new and desire to secure by Letters Patent of the United States is:

1. In a window assembly for providing a vacuum tight barrier between two rectangular waveguide systems which assembly permits transmission of electromagnetic waves, a first waveguide section of rectangular cross section, a second waveguide section of rectangular cross section coaxially poistioned relative to said first segment and a closure between said waveguide sections having two broad walls extending from corresponding walls of said first waveguide section, said closure forming a vacuum tight barrier therebetween and extending into said second waveguide section, said closure having at least one aperture in each broad wall for transmitting electromagnetic energy between said first and second waveguide sections and microwave windows of a dielectric material closing the apertures.

2. A microwave window for providing a vacuum tight barrier between waveguide sections and providing for the transmission of electromagnetic energy therebetween including first and second coaxially positioned waveguide sections of rectangular cross section each having broad and narrow walls and a vacuum tight closure between said sections, said closure comprising a pair of sloping side plates extending inwardly from the broadwalls of said first waveguide and intersecting within said second waveguides and a pair of conductive plates positioned at opposite edges of said sloping plates and forming a vacuum tight seal defining a conductive nose section having the shape of a gable roof with the internal angle closed, each sloping side of said closure having at least one aperture therein for transmitting electromagnetic energy and microwave windows of dielectric material sealing the apertures in said sloping sides of said closure.

3. A waveguide window section including a waveguide of rectangular cross section with narrow walls and broad walls, a microwave window and closure section for said waveguide comprising a conductive nose section having the shape of a gable roof with the internal angle closed, each sloping side of said closure extending inwardly from a broad wall of the waveguide and having at least one aperture therein for transmitting electromagnetic energy, and dielectric microwave windows sealing the apertures in said sloping sides of said closure to form vacuum tight barrier.

4. A waveguide window section of rectangular cross section divided into first and second waveguide sections by a vacuum tight barrier sealed therein, said vacuum tight barrier having two broad walls extending from corresponding walls of said first waveguide section and extending into said second waveguide section and having at least one aperture in each broad wall for transmitting electromagnetic energy through the window section, and dielectric microwave windows positioned and sealed in the apertures to form vacuum tight closures.

5. A waveguide window section including a waveguide of rectangular cross section with narrow walls and broad walls; a microwave window and closure section for said waveguide comprising a pair of sloping side plates extending inwardly from the broad walls of said waveguide and having at least one aperture in each plate for transmitting electromagnetic energy, means for closing the area between the sloping edges of said sloping plates and dielectric microwave windows sealing the apertures in said sloping sides of said closure to form a vacuum tight barrier, whereby said window and closure section defines a conductive nose section having the shape of a gable roof with the internal angle closed; and a fiat conductive late positioned within said nose section and bi-secting the internal angle between said sloping sides to form a septum in said window section.

6. A waveguide section including a waveguide of rectangular cross section with narrow walls and broad walls; a microwave window and closure section for said waveguide comprising a pair of sloping sides of said closure extending inwardly from a broad wall of the Waveguide and each sloping side having at least one aperture therein for transmitting electromagnetic energy, means for closing the area between the sloping edges of said sloping plates, and dielectric microwave windows sealing the apertures in said sloping sides of said closure to form a a vacuum tight barrier, whereby said window and closure section defines a conductive nose section having the shape of a gable roof with the internal angle closed; and a flat conductive plate positioned within said nose section and bi-secting the internal angle of said hip and extending therethrough to form a septum in said Window section.

7. A microwave window section for connecting two rectangular waveguide systems and providing a vacuum tight seal therebetween which is capable of transmitting electromagnetic energy including a first waveguide of rectangular cross section, a second waveguide of rectangular cross section and larger internal dimensions than the external dimensions of said first waveguide, said first waveguide extending into said second waveguide section and being provided with at least one coupling aperture in each broad Wall, said coupling apertures all being located inside said second waveguide section and end closure plates on the end of said first waveguide section which extends inside said second waveguide, a dielectric microwave window positioned and sealed in within each coupling aperture, and conductive support means extending between the end of said second waveguide section which surrounds said first waveguide section and said first waveguide section to form a rigid window section.

References (Iited in the file of this patent UNITED STATES PATENTS 2,684,469 Sensiper July 20, 1954 2,791,720 Lesch May 7, 1957 2,852,752 McCreary Sept. 16, 1958 2,879,485 Carter et al Mar. 24, 1959 2,894,228 Geisler July 7, 1959 FOREIGN PATENTS 584,895 Canada Oct. 13. 1959 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,039,068 June 12, 1962 Norman A. Greco It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 17, for "passhoard" read passband line 19, for "strike" read strive column 6, line 71, strike out "a".

Signed and sealed this 6th day of November 1962.

(SEAL) Attest:

ERNEST w. SWIDER DAVID L LADD Attesting Officer Commissioner of Patents 

